The correct option is A We must catch the 5 o' clock train and there being only half an hour left, we must start without further delay.
Cumulative conjunctions are used for merely adding one statement to another.
Option A is correct: The given sentences are joined using "and" which is an example of cumulative conjunction.
Option B is incorrect: "As" is used to give reasons. Hence, it is incorrect.
Option C is incorrect: "With" is used to show contrast. Further the sentence structure of the statement is incorrect. Hence, it is incorrect
Option D is incorrect: Since, Option A is correct, this cancels out Option D.